This invention is related to automotive powertrain systems including computer control of various engine and automatic transmission functions. More specifically, the invention is concerned with control of torque during ratio changes of the automatic transmission.
Generally, it is desirable to perform ratio changes in an automatic transmission such that the total shift time remains within a predetermined range to provide for consistent quality shift feel. Shifts of longer and shorter durations may result in excessive wear of torque transmitting friction devices and undesirably harsh shift feel, respectively, as well as other undesirable effects. A major factor affecting shift duration is the torque at the friction device being applied (shift torque) which may vary significantly in accordance with such performance demands as acceleration and vehicle loading due to concomitant increases in engine torque. Known powertrain control systems seek to equalize the diverse shift torques to a target shift torque during ratio changes at these various performance demands by reducing the engine torque to a predetermined level. The resultant shift torque is utilized to set the shift pressure for use during the shift, which pressure in turn establishes the shift duration.
However, other factors not related to performance demands may affect the stability of the shift times. Barometric pressure for example may significantly affect the engine torque produced at a given throttle setting. Also, different shift schedules, for example for performance and economy shift points, may affect the shift torque. These factors, among others, may affect the proportional balance among various torque constituents comprising shift torque. Therefore, a constant percentage reduction in engine torque for a given shift torque still may result in very different shift torques at certain operating conditions.